1. Field of the Invention
The invention relates to a system for locating a plurality of objects and obstructions and for detecting and determining the rolling status of moving objects, such as aircraft, ground vehicles. and the like, in the region of an airport, comprising a plurality of transmitting and receiving stations (radar stations) and a central station.
2. Description of the Prior Art
Due to improvements in instrument landing systems (ILS), approaches and landings at airports under poor weather conditions no longer present any problems. However, real problems are evident in the subsequent taxiing or rolling phase between runways and gates. With high traffic density, nighttime darkness and/or poor weather conditions, tower controllers do not know the exact position of all aircraft or ground vehicles, such as the so-called follow-me cars, at the airport, and furthermore, pilots do not know their exact position at the airport. Due to poor visibility, obstructions, such as detached engine cowlings, lost luggage trailers, stray service vehicles, and aircraft, represent a great danger.
Until now the ground traffic control is regulated by visual observation of the situation by controllers and pilots. With poor visibility it is therefore necessary to greatly reduce the traffic density. Aircrafts are guided at critical points by optical signals and radiotelephony.
At some major airports, so-called airport monitoring primary radar equipment is also available, and is referred to as airport surface detection equipment (ASDE). Even when this radar is mounted on a tower, shadows of aircrafts with respect to one another and particularly, shadows caused by buildings occur in significant regions of the airport area. Due to multipath propagations of the radar signals, false targets may also be detected. Moreover, the direct determination of the speed vector of individual objects is not possible. It is likewise impossible to determine the direction of a vehicle's longitudinal axis and its identity.
In DE 29 34 844 A1, a cooperative method, in the form of an airport monitoring system, is described. For monitoring only one runway of an airport with a length of, for example, 4 km, a large number of ground devices are required. Location is possible only within the order of magnitude of the mutual distances of the sensors; The actual rolling status, i.e. position, velocity vector, vehicle alignment and identity can be detected only to a very limited extent. Classification and determination of aircraft alignment is not possible at all.
The secondary surveillance radar (SSR) is presently used in aviation solely for monitoring flight traffic from the ground. Aircraft position is determined by measuring the slant range and the azimuth angle. Furthermore, when activated by an interrogation signal from the SSR, the aircraft transponder transmits a code for distance measurement and identification purposes. To increase the capacity of the system, the new mode S has been defined and standardized. Introduction thereof has started in the USA. In contrast to normal SSR, some features of the SSR mode S make it very suitable for ground traffic control as well.
The problems in cooperative methods, and thus, in cooperative rolling status detection (RSD), reside in very few aircraft and ground vehicles being equipped with mode S transponders in the next few years. Even in the remote future of general aviation, a great number of aircraft will not have this type of transponder.
To increase the capacity of an airport, and in particular, to maintain safety for rolling traffic under poor weather conditions, new methods must be developed and introduced for ground traffic control to overcome the difficulties mentioned above. An essential component of future ground traffic control systems will be sensor systems for detecting the rolling status of aircraft and vehicular traffic and the presence of any obstructions on the ground at the airport. The rolling status is made up of a vehicle's position, its speed vector, the direction of the vehicle's longitudinal axis, its so-called heading, and its identity.
For locating obstructions and for detecting the rolling status of objects such as aircraft, ground vehicles, and the like, which do not have a mode S transponder, a so-called non-cooperative method will always be necessary, such as, for instance, primary radar techniques in which the scattering of electromagnetic waves at objects is utilized for location. A further important aspect of the primary radar technique is that it can be implemented independently of international standardizing steps. The cooperative and non-cooperative rolling status detection methods do not conflict, but rather supplement each other in an expedient manner.
In D E 21 44 533 B2, a radar traffic surveillance system is described for airports in particular, that monitors the landing area, the runways and the taxiways. This surveillance system represents a non-cooperative method which can monitor only a single runway. Surveillance of extended areas, such as the entire airport or even only the so-called airport apron in front of the terminals, cannot be conducted with this system. In these areas it is not possible to monitor several objects simultaneously. In addition, fixed objects are not detected at all and determination of the rolling status is possible only to a very restricted degree.
Furthermore, in DE-AS 1,240,950, an air traffic surveillance system is known, which likewise represents a non-cooperative method, and like the surveillance system described above, cannot cope with multiple objects. Consequently, it is not being used for ground traffic control. A separation of targets is possible only in the specific case where aircrafts are spatially well separated from each other in the air space, e.g., different airways.
FIG. 5 is a simplified representation of the area of an airport taxiway with only two vehicles 01 and 02 in the form of aircrafts. Location of aircraft position is performed by means of two radar stations R1 and R2 with antennas AR1 and AR2 associated therewith. It is known that objects 01 and 02 can be located by monostatic echo delay time measurements from two radar stations R1 and R2 in which the transmitting and receiving antennas are both at the same location, or by bistatic echo delay time measurements from two radar stations in which the transmitter and receiver are spatially separated. Nevertheless, these arrangements are not suitable for locating a large number of objects (for example a few hundred objects), as must be monitored in the surveillance area of a major airport, because these arrangements cannot detect multiple objects.
Since all objects within an illuminated region which are located in the same range zone, as represented in FIG. 5 by concentric circular segments, contribute to a backscatter signal of said range zone, erroneous locations occur even with just two objects (e.g. in the form of the objects 01 and 02). In FIG. 5, these false objects are denoted by "X". On airport aprons where many objects can have practically any position, the location results from employing two radar stations only can therefore be referred to as "chaotic".